This invention pertains generally to pattern recognition, and in particular to a method of classifying objects by matching the silhouette of an unknown object to the silhouette of a known object.
Object recognition may be performed using any distinctive characteristic of the object. One such characteristic useful in many cases is the silhouette of the object as recorded by a device sensitive to visible light or infrared radiation. If the silhouette of an unknown object can be matched to the silhouette of a previously identified object, it may be concluded that the unidentified object is of the same class as the known object. Since the silhouettes are equivalent to two-dimensional patterns, any known pattern-matching technique may be used.
In one known technique, the silhouettes are divided into small elements, or "pixels." The known and unknown silhouettes are deemed to be matched if the contents of a predetermined percentage of corresponding pixels are the same. The technique being discussed, though computationally simple, performs poorly if the silhouettes being compared are not in registration one with the other. The technique also performs poorly if the image of the unknown object is a different size than that of the image of the known object (as might be the case if the images were made by devices different distances from the object). Another technique, which involves comparing two-dimensional Fourier transforms of a known image with the transform of an unknown image, operates well even if the known and unknown silhouettes are oriented differently. However, such a technique requires substantially more computation.